Artificial lighting derived from alternating current sources, particularly fluorescent lighting, contains a strong brightness modulation component, or flicker, at twice the frequency of the alternating current sources. This factor of 2 arises from the power relation between instantaneous voltage of the alternating current sources and instantaneous brightness, and from the trigonometric relation cos2 (x=0.5(1+cos(2x)). Commonly encountered flicker frequencies are 100 Hz in Europe and 120 Hz in the United States. Although invisible to the human eye, flicker may be highly visible to image sensors. The problem is most apparent at low exposure values. An imagine sensor samples this modulation waveform as reflected from objects in the scene and reproduces it perfectly.
Solid-state sensors fall into two broad categories according to exposure method. One category is full-field, where all pixel elements of the sensor are exposed simultaneously. A second category is rolling window, where all pixel elements in a sensor row are exposed simultaneously, but the onset of exposure is delayed from row to row. Lighting flicker induces a periodic variation in luminance, known as banding. Banding is apparent in the time domain, and in the case of rolling-window sensors banding is also apparent in the vertical spatial domain.
In the case of the rolling-window sensors, should the camera and the frequency of the alternating current source be in perfect synchronization, the modulation pattern will be temporally frozen, appearing as static luminance banding down the image. However, the problem is compounded if camera field rates and frequency of the alternating current source differs by some amount, causing the luminance modulation bands to roll up or down the image. The rate of roll depends mostly on whether the camera is operating home or away, i.e., nominal frame rate may be a close sub-multiple of the frequency of the alternating current source. For example, a 50 Hz camera operating in the United States is operating away. Roll associated with a camera operating at home is extremely slow, while roll associated with a camera operating away is much faster.
As well as being visibly distracting to the viewer, luminance modulation generates considerable frame-to-frame differences in image streams which could, for example, make the difference between a software video CODEC performing acceptably. Thus, it is important that a camera system be capable of detecting and cancelling artificial lighting flicker.
Detection of lighting flicker in the spatial domain is difficult in the case of rolling-window exposure sensors, and is much more difficult in the case of full-field exposure sensors. In the former case the difficulty is due to potential strong correlations between expected banding patterns caused by lighting flicker and variations in actual scene luminance.
U.S. Pat. No. 5,053,871 discloses a still video camera which uses a previewing technique to provide automatic exposure control and flicker detection. However, there is a need to provide flicker detection in motion video cameras. U.S. Pat. No. 5,272,539 discloses a video camera with flicker detection, but in this prior arrangement the detector frame rate is coupled with the video frame rate, which limits its usefulness.